Method of operating electron multipliers



Nov. 14, 1939. T FARNSWORTH 2,180,279

METHOD OF OPERATING ELECTRON MULTIPLIERS Filed, March 22, 1957 IN VENTOR. PH/LO T. FAPNSWOPTH.

ATTORNEYS.

Patented Nov. 14, 1939 UNITED STATES PATENT OFFICE METHOD OF OPERATINGELECTRON MULTIPLIERS ware Application March 22, 1937, Serial No. 132,329

6 Claims.

' and more particularly to a method of operating electron multipliersenergized by alternating current.

Among the objects otmy invention are: to

provide a novel method of operating electron.

multipliers energized by alternating current; to provide alternatingcurrent of special wave form for the operation of electron multipliers;and to provide a method of reducing shot effect in an electronmultiplier.

My invention possesses numerous other objects and features of advantage,some of which,'together with the foregoing, will be set forth in 5 thefollowing description of specific apparatus embodying and utilizing mynovel method. It is therefore to be understood that my method isapplicable to other apparatus, and that I do not limit myself, in anyway, to the apparatus of the present application, as I may adopt variousother apparatus embodiments, utilizing the method, within the scope ofthe appended claims.

Referring to the drawing:

Figure 1 is a graph illustrating the electron multiplication in anelectron multiplier supplied with a-voltage of sinusoidal wave form.

Figure-2 is a diagram, reduced to the simplest terms, of a diodeelectron multiplier and circuit.

Figure 3 is a graph showing-a special wave form which may be used inconjunction with the energization of the device of Figure 2.

Figure 4 is a diagrammatic drawing and circuit, in simplified form, ofanother modification of an electron multiplier and circuit.

Figure 5 is a drawing of a wave form which may be used in conjunctionwith the device of Figure 4.

In my prior applications, Serial No. 692,585,

filed October-7, 1933, Patent No. 2,071,515, dated 9 February 23, 1937,and Serial No. 61,042, filed January 27, 1936, Patent No. 2,137,528,dated November 22, 1938, I have described the operation of electronmultipliers wherein voltages of sinusoidal wave form are used toenergize the 4,5 cathode and anode. The fundamental principle involvedin these electron multipliers isthat electrons are oscillated torepeatedly impact a surface capable of generating secondary electrons ata ratio greater than unity. The energy for 50 causing the oscillation ofthe electrons may be supplied by an outside source or the device may bemade to self-oscillate, thus supplying its own alternating potential.When a source of electrons is provided external to the multiplier and 55it is desired to multiply electrons from that (01.250-27) My inventionrelates to electron multipliers source, it is common to direct theseelectrons to be multiplied through an aperture in the device so thatthey may come within the influence of the oscillating fields.

' In this latter case, it is customary to drive the 5 device rather thanallow it to self-oscillate, in

order that substantially linear amplification may be obtained. Further,to obtain linear amplification, it is highly desirable that each of theincoming electrons be multiplied by the same num- 10 her of impacts asevery other incoming electron.

= Otherwise a shot effect is produced and perfect linear amplificationis not obtained. My present invention comprises a method of reducing theshot effect by using'a non-sinusoidal driver. 15

My novel method may be more easily understood by reference to thedrawing, which is diagrammatic in character and which will serve toilllustrate my method graphically.

Referring first to Figure 2, which shows a com- 20 mon A. C. multipliercircuit with a sinusoidal driver, an envelope I, is provided as a partof an envelope of another device, not shown, wherein electrons areemitted into space and thereafter travel into-the envelope I. Theincoming elec- '25 trons to be multiplied are represented as arrivingalong a direction indicated by arrow 2. The

multiplier itself, in this case, comprises the wellknown Farnsworthdiode having an exterior cathode cylinder 4 and an interior apertured 0.anode 5 in the electron path. The cathode cylinder 4 has its innersurface sensitized or otherwise treated so that electrons, will beproduced at a ratio greater than unity when impacted by an electrontraveling, for ex- 35 ample, with a velocity of 20 electron volts ormore. Multiplication ratios up to 1 to 12 may be obtained.

An aperture 5 is provided in the cathode 4 through which the electronsto be multiplied may 40 enter the multiplier structure itself. It isobvious that the size of this aperture may be varied in accordance wtihthe size of the electron beam.

The electrodes themselves are energized by being connected through aseries circuit comprising a resonant circuit 1 and a series condenser 9.Further, the cathode is connected to the negative terminal of abattery8, the positive terminal of which is at ground potential. Theanode 5 is provided with a steady positive accelerating potential withrespect to the cathode 4 by connecting one side of the tuned circuit toground, thus completing the connection to the anode,

.as indicated in Fig. 2. .The output may be taken off across theresonant circuit 1. T

A driver oscillator I is coupled to the tuned circuit 1. I prefer to soadjust the constants of the tuned circuit 1 and the value of theaccelerating potential between the cathode 4 and 5 the anode 5 that morethan one electron impact may take place per cycle. If the anode voltagewith respect to ground which is represented by the line g-t of Figure lis plotted against time,

a sinusoidal voltage wave A-E will appear. The distance between thiscurve and the horizontal cathode voltage line at the bottom of the graphof Figure 1 represents the difference of potential between anode andcathode, as shown in Figure 1. The operation of the device operatingwith such a sinusoidal potential will now be described.

If we assume the steady difference of potential between anode andcathode to be 1000 volts and the alternating potential difference to be800 volts, an electron leaving the cathode at point A will beaccelerated by about 1800 volts. During the time elapsing between theentry of the electron and its impact with the cathode, which may becalled the flight time of the electron required for one trip ormultiplication, the total potential difference decreases and theelectron returns and strikes the cathode at the end of the trip with avelocity equal to the decrease in potential difference during its timeof flight. As the difference of potential between anode and cathode isat its maximum at point A, the maximum acceleration of the electronstakes place in this part of the radio-frequency cycle, and hence theshortest time of flight occurs. The voltage, however, has changed verylittle between point A and point B; thus the multiplication ratioobtained will not be very great. As the potential decreaseathe times offlight become longer and the potential differences between start and endof the trips are therefore greater; hence the multiplication ratiosbecome larger.

Referring now to electrons to be multiplied entering the multiplierchamber, it is obvious that an electron entering the multiplier chamberat time A will make the greatest number of flights, and hence maximummultiplication will be obtained. However, an electron entering themultiplier a short time later at time B will make one less trip andtherefore lose one multiplication, although the loss of the first tripwill not be serious because very little multiplication takes place atthe first impact. The loss of multiplication, however, becomes more andmore serious if an electron enters after a few trips have been made,particularly as the ratio of multiplication is continuously increasing.Thus, it may be seen that the electrons entering the multiplier over theentire multiplying period of the radio-frequency cycle from A to Eexperience a multiplication, but .the ratio of multiplication differstremenduously with the time at which the electron enters the multiplier.

While this shot effect is not a great disadvantage under certainconditions, particularly where there is a profuse supply of incomingelectrons, it is obvious that if the incoming electrons to be multipliedare relatively few in number, an output may be obtained which in no wayperfectly resembles the input.

I have, however, devised a method such that substantially the same ratioof multiplication will be obtained for all electrons entering themultiplier during the greater part of the radio-frequency cycle.

To obtain this result, I drive the multiplier with a voltage ofnon-sinusoidal wave shape, one

preferred wave form being shown graphically in Figure 3. With thisvoltage wave, electrons entering the multiplier during the interval between times A and B are multiplied in the ratio of approximately one toone. This period may be conveniently called an accumulation period, andit should take up the greater portion of the radio-frequency cycle inorder to fulflll the purpose of substantially equal multiplication ofall electrons. Under this mode of operation an electron, starting fromthe cathode at time A, undergoes the maximum acceleration. The time offlight, therefore, is very short. During this time of flight the voltagemay have fallen only about fifteen volts and after allowing forty trips,for example, the voltage will have dropped only 600 volts. So as not tolose too much of the acceleration voltage, the trips in the accumulationperiod between A and B are limited to a certain number. The voltagebetween B and C, however, decreases rapidly. The number of secondaryelectrons per impact will therefore increase up to approximately ten,for example.

If the time of flight is made short enough in comparison with theradio-frequency cycle, it is possible for the electrons to make three orfour trips in the interval from B to C, utilizing the remainder of thepotential difference. In other words, the radio-frequency cycle isdivided into an accumulation period from A to B, and a multiplyingperiod from B to C; collection takes place at all times, but collectionalone takesplace in the period from C to D.

Thus there is only a small portion of the radiofrequency cycle wherehigh multiplication ratios are obtained, and therefore the shot effectwill be greatly reduced.

So far, I have described my novel method as applied to the cylindricaldiode structures shown in Figure 2. If, however, the type of multipliershown in Figure 4 is utilized, having an anode not in the electron path,an even more advan tageous mode of multiplication may be obtained. Inthis type of. structure the cathode 4 is in the form of two opposedplates, one of which is provided with aperture 6 in the path 2 of theelectrons to be multiplied, and in this case the accelerating anode 5 isin the form of a ring surrounding the space between the two cathodes. A

, solenoid II is provided around the tube and is energized by solenoidsource l2 so that a strong magnetic field perpendicular to the cathodesurfaces is obtained. This magnetic field prevents the electrons frombeing immediately collected by the anode ring 5, and they will not becollected until their velocity is low.

Generator I0 is then connected to energize the anode and the twocathodes connected together, and drives the device with a voltage ofnonsinusoidal wave form of the shape, for example, shown in Figure 5.The usual steady acceleration potential is provided on the anode. Theoutput may be taken oil across the resonant cir- 'cuit 1.

In this case, an electron to be multiplied enters the multiplicationchamber through cathode aperture 6 at time A on the voltage wave showncreased again. Thus, the entering electrons travel back and forthwithout striking the oathodes, nor are they collected by the anode 5.The maximum potential difierence is at time B, and the time of flight,therefore, will be the shortest at that time. Beyond point B in the timecycle, the electrons will be able to strike the cathodes as thepotential difference rapidly decreases. Multiplication, therefore, takesplace at high ratios until the potential difference reaches its minimumvalue at time C. From to D the electrons are decelerated and collectionoccurs by the anode 5.

This mode of operation has an advantage over the previously describedmode in that the entire swing of the potential difference can be madeuseful for the multiplication of the accumulated electrons.

Furthermore, all electrons accumulated between times A and B in thislatter instance are multiplied by the same ratio, namely, the ratio of.multiplication when impacting the first cathode surface after entry intothe multiplier.

It is to be noted that as connected both multiplier structures areoperated as diodes, the only difference being in the use of an anodeinFigure 2 positioned in the electron path, and an anode intFigure 4positioned outside of the electron pa h. I

Summarizing, it may be seen that what I have done is to accumulateelectrons with substantially uniform, low multiplication during a largepart of a multiplying cycle, and then to multiply them at high ratiosduring an extremely short part of the multiplying cycle. Thus I havebeen able to obtain a great diminution in the shot effect and have beenable to give a far more equal multiplication of electrons entering amultiplier during a given period of time than can be obtained by the useof a voltage of sinusoidal wave form to energize the multiplier.

It is further to be understood that the particular wave forms hereexhibited are illustrative only and first while I have shown noparticular apparatus for obtaining these wave forms, it is well known inthe art how to modify sinusoidal waves or to provide special oscillatorsin order to obtain wave forms of practically any shape desired. As suchspecific apparatus is no part of the present invention, it will not bedescribed in this application.

It is also obvious that I do not desire to be limited to any particularsource of incoming electrons to be multiplied, nor does my method Inecessitate any outside source. For example, the. cathode may bephoto-electric and itself generate, under the influence of light, theinitial electrons. My method, therefore, is applicable to allarrangements wherein multiplication of elecratio multiplication for arelatively long portion of the cycle and then to a relatively high-ratiomultiplication for a relatively short portion of the cycle.

3. In an electron multiplier wherein electrons are oscillated againstand away from a surface adapted to emit secondary electrons uponelectron impact therewith, the method of operation which comprisesoscillating the electrons by the application of a slowly changing fieldto produce low-ratio multiplication, and continuing the oscillation bythe application of a rapidly changing field to produce high-ratiomultiplication.

4. In an electron multiplier wherein electrons are oscillated againstand away from a surface adapted to emit secondary electrons uponelectron impact therewith, the method of operation which comprisesoscillating the electrons by the application of a slowly changing fieldto produce low-ratio multiplication, continuing the oscillation bytheapplication of a rapidly changing field to produce high-ratiomultiplication, and

collecting the electrons after said high-ratio multiplication. a

5. In an electron multiplier wherein electrons are oscillated againstand away from a surface adapted to emit secondary electrons uponelectron impact therewith, the method of operation which comprisesoscillating the electrons without impact for a predetermined time andthereafter continuing theoscillation with impact to producemultiplication.

6. In an electron multiplier wherein electrons are oscillated againstand away from a surface adapted to emit secondary electrons uponelectron impact therewith, the method of. operation which comprisessupplying electrons in a continuous. stream, accumulating electrons byoscillating the electrons without impact for a predetermined -time andthereafter continuing the oscillation with impact to producemultiplication.

- PHILQ T. FARNSWORTH.

